Synthesis, characterization and biological properties of thienyl derived triazole Schiff bases and their oxovanadium(IV) complexes

Abstract

A new series of biologically active thienyl derived triazole Schiff bases and their oxovanadium(IV) complexes have been synthesized and characterized on the basis of physical (m.p., magnetic susceptibility and conductivity), spectral (IR, ¹H and ¹³C NMR, electronic and mass spectrometry) and microanalytical data. All the Schiff base ligands and their oxovanadium(IV) complexes have been subjected to in vitro antibacterial activity against four Gram-negative (Escherichia coli, Shigella flexneri, Pseudomonas aeruginosa, Salmonella enterica serover typhi) and two Gram-positive (Staphylococcus aureus and Bacillus subtilis) bacterial strains and, for in vitro antifungal activity against Trichophyton longifucus, Candida albican, Aspergillus flavus, Microscopum canis, Fusarium solani and Candida glabrata. Brine shrimp bioassay was also carried out to check the cytotoxic nature of these compounds.

Keywords::

• 1;2;4-triazole
• thiophene-2-carboxaldehydes
• oxovanadium(IV)
• antibacterial
• antifungal

Introduction

Compounds having S, N and O donor atoms play a vital role in antibacterial^1–3, antifungal^4,5, antitumor⁶, antitubercular^7,8, anticancer⁹ and antiviral¹⁰ activities due to their significant biological properties. Presently, the vanadium metal chemistry is gaining attention for its remarkable activities such as antimicrobial¹¹, antitumor¹², and most recently as insulin mimetic¹³. In continuation of our previous efforts to design and synthesize biologically active compounds, we have prepared a new series of thiophene derived triazole Schiff base ligands (L¹)–(L⁵) and their oxovanadium(IV) complexes (1)–(5) (Scheme 1) and wish to report in this paper their characterization and in vitro antibacterial, antifungal and cytotoxic properties.

Scheme 1.  Preparation of ligands (L¹)-(L⁵) and their oxovanadium(IV) complexes (1)–(5).

Materials and methods

All the chemicals used for preparation of the triazole Schiff base ligands and their oxovanadium(IV) complexes were of analytical grades. Melting points were recorded on Fisher Johns equipment. IR spectra were recorded on Shimadzu FTIR spectrophotometer. Elemental analysis (C, N, H) was carried out on Perkin-Elmer (USA model). ¹H and ¹³C NMR spectra were recorded on a Bruker Spectrospin Avance DPX spectrometer at 400 MHz, respectively. Electron impact mass spectra (EIMS) were recorded on JEOL MS Route instrument. Stanton SM12/S Gouy’s balance was used to measure magnetic susceptibility of the oxovanadium complexes at room temperature. Molar conductance was taken by Inolab Cond 720 Conductivity Bridge. In vitro antibacterial, antifungal and cytotoxic properties were studied at HEJ Research Institute of Chemistry, International Centre for Chemical Sciences, University of Karachi, Pakistan.

Synthesis of triazole derived Schiff base (L¹)

An equimolar methanol solution of 3,5-diamino-1,2,4-triazole (0.99 g, 10 mmol, 20 mL) and thiophene-2-carboxaldehyde (0.94 mL, 10 mmol, 20 mL) was stirred at room temperature for 2 h and the reaction was monitored by TLC. Precipitation took place during stirring. It was then filtered, washed with methanol, then with diethyl ether and dried under vacuum. Recrystallization was carried out in a mixture of methanol:dioxane (1:4) to get required TLC pure product (L¹) in 70% yield. The same method was applied for the preparation of other ligands (L²)–(L⁵) (Scheme 1).

N³-[(E)-thiophen-2-ylmethylidene]-1H-1,2,4-triazole-3,5-diamine (L¹)

Yield (1.35 g, 70%); m.p. 172–174°C; IR (KBr, cm⁻¹): 3345 (NH₂), 3190 (NH), 1631 (HC=N), 1609 (C=N, triazole), 1570, 1546 (C=C), 1020 (N-N), 882 (C-S); ¹H NMR (DMSO-d₆, δ ppm): δ 6.05 (s, 2H, NH₂), 7.20 (dd, 1H, J = 4.9, 3.8 Hz, thienyl C₄-H), 7.71 (d, 1H, J = 4.9 Hz, thienyl C₃-H), 7.81 (d, 1H, J = 3.8 Hz, thienyl C₅-H), 8.32 (s, 1H, C₆-H), 11.92 (s, 1H, NH); ¹³C NMR (DMSO-d₆, δ ppm): δ 125.22 (C₅), 133.54 (C₄), 138.72 (C₃), 144.31 (C₂), 156.12 (C₆), 160.97 (C₈), 163.98 (C₇); EIMS (70 eV) m/z (%): 193.2 ([M]⁺, 100), 177 (5), 137 (34), 122 (22), 110 (10), 96 (21), 84 (13); Anal. Calcd. for C₇H₇N₅S (193.23): C, 43.51; H, 3.65; N, 36.24; S, 16.59; Found: C, 43.49; H, 3.61; N, 36.21; S, 16.55%.

N³- [(E)-(5-methylthiophen-2-yl)methylidene]-1H-1,2,4-triazole-3,5-diamine (L²)

Yield (1.59 g, 78%); m.p. 234–236°C; IR (KBr, cm⁻¹): 3344 (NH₂), 3187 (NH), 1633 (HC=N), 1609 (C=N, triazole), 1578, 1556 (C=C), 1018 (N-N), 884 (C-S); ¹H NMR (DMSO-d₆, δ ppm): δ 2.45 (s, 3H, CH₃), 5.97 (s, 2H, NH₂), 6.90 (d, 1H, J = 3.8 Hz, thienyl C₄-H), 7.50 (d, 1H, J = 3.6 Hz, thienyl C₃-H), 8.41 (s, 1H, C₆-H), 11.86 (s, 1H, NH); ¹³C NMR (DMSO-d₆, δ, ppm): δ 15.62 (CH₃- thienyl), 126.97 (C₄), 134.61 (C₃), 140.15 (C₅), 145.89 (C₂), 156.39 (C₆), 161.27 (C₈), 163.54 (C₇); EIMS (70 eV) m/z (%): 207 ([M]⁺, 100), 192 (5), 151 (22), 136 (12), 123 (9), 110 (13), 109 (9), 84 (7); Anal. Calcd. for C₈H₉N₅S (207.26): C, 46.36; H, 4.38; N, 33.79; S, 15.47; Found: C, 46.36; H, 4.35; N, 33.76; S, 15.44%.

N³-[(E)-(3-methylthiophen-2-yl)methylidene]-1H-1,2,4-triazole-3,5-diamine (L³)

Yield (1.43 g, 69%); m.p. 239–240°C; IR (KBr, cm⁻¹): 3343 (NH₂), 3187 (NH), 1629 (HC=N), 1610 (C=N, triazole), 1575, 1562 (C=C), 1020 (N-N), 885 (C-S); ¹H NMR (DMSO-d₆, δ ppm): δ 2.22 (s, 3H, CH₃), 6.04 (s, 2H, NH₂), 7.0 (d, 1H, J = 3.8 Hz, thienyl C₄-H), 7.68 (d, 1H, J = 3.7 Hz, thienyl C₅-H), 8.42 (s, 1H, C₆-H), 11.88 (s, 1H, NH); ¹³C NMR (DMSO-d₆, δ, ppm): δ 16.78 (CH₃- thienyl), 122.63 (C₂), 126.39 (C₄), 135.54 (C₅), 143.77 (C₃), 156.93 (C₆), 160.85 (C₈), 163.85 (C₇); EIMS (70 eV) m/z (%): 207.2 ([M]⁺, 100), 192 (5), 174 (4), 165 (5), 151 (19), 136 (11), 134 (14), 124 (13), 115 (15), 109 (18), 84 (5); Anal. Calcd. for C₈H₉N₅S (207.26): C, 46.36; H, 4.38; N, 33.79; S, 15.47; Found: C, 46.31; H, 4.36; N, 33.76; S, 15.41%.

N³-[(E)-(5-chlorothiophen-2-yl)methylidene]-1H-1,2,4-triazole-3,5-diamine (L⁴)

Yield (1.69 g, 74%); m.p. 224–225°C; IR (KBr, cm⁻¹): 3346 (NH₂), 3186 (NH), 1636 (HC=N), 1612 (C=N, triazole), 1567, 1544 (C=C), 1022 (N-N), 887 (C-S), 810 (C-Cl); ¹H NMR (DMSO-d₆, δ ppm): δ 6.07 (s, 2H, NH₂), 7.33 (d, 1H, J = 2.8 Hz, thienyl C₄-H), 7.73 (d, 1H, J = 2.6 Hz, thienyl C₃-H), 8.53 (s, 1H, C₆-H), 12.15 (s, 1H, NH); ¹³C NMR (DMSO-d₆, δ, ppm): δ 124.43 (C₃), 126.82 (C₄), 132.36 (C₅), 143.39 (C₂), 157.22 (C₆), 161.12 (C₈), 164.12 (C₇); EIMS (70 eV) m/z (%): 228.3 ([M]⁺, 100), 192 (31), 171 (24), 158 (6), 144 (5), 130 (10), 119 (13), 95 (8), 84 (16); Anal. Calcd. for C₇H₆ClN₅S (227.67): C, 36.93; H, 2.66; N, 30.76; S, 14.08; Found: C, 36.91; H, 2.64; N, 30.71; S, 14.05%.

N³-[(E)-(5-nitrothiophen-2-yl)methylidene]-1H-1,2,4-triazole-3,5-diamine (L⁵)

Yield (1.61 g, 68%); m.p. 188–189°C; IR (KBr, cm⁻¹): 3349 (NH₂), 3194 (NH), 1637 (HC=N), 1611 (C=N, triazole), 1565, 1552 (C=C), 1355 (NO₂), 1024 (N-N), 889 (C-S); ¹H NMR (DMSO-d₆, δ, ppm): δ 6.09 (s, 2H, NH₂), 7.94 (d, 1H, J = 3.4 Hz, thienyl C₃-H), 8.04 (d, 1H, J = 3.2 Hz, thienyl C₄-H), 8.68 (s, 1H, C₆-H), 12.20 (s, 1H, NH); ¹³C NMR (DMSO-d₆, δ, ppm): δ 129.39 (C₃), 134.43 (C₄), 145.17 (C₅), 149.97 (C₂), 157.48 (C₆), 161.88 (C₈), 166.24 (C₇); EIMS (70 eV) m/z (%): 238.2 ([M]⁺, 100), 192 (95), 182 (11), 157 (6), 150 (19), 136 (8), 123 (5), 109 (12), 95 (20); Anal. Calcd. for C₇H₆N₆O₂S (238.23): C, 35.29; H, 2.54; N, 35.28; S, 13.46; Found: C, 35.26; H, 2.51; N, 35.25; S, 13.41%.

Synthesis of oxovanadium(IV) complexes

Oxovanadium(IV) complex with (L¹) (1)

To a hot magnetically stirred dioxane solution of N³-[(E)-thiophen-2-ylmethylidene]-1H-1,2,4-triazole-3,5-diamine (L¹) (0.39 g, 2 mmol, 30 mL), a methanol solution of vanadyl(IV) sulphate (0.163 g, 1 mmol, 20 mL) was added. The mixture was refluxed for 3 h and then cooled to room temperature. During the refluxing, precipitation took place. The precipitated product thus formed was filtered off, washed with methanol, dioxane and then with diethyl ether. It was recrystallized in a mixture of water:dioxane (1:3) to obtain the pure required complex (1). All other complexes (2)–(5) were prepared following the same method using the same vanadyl salt with the respective Schiff base ligands (Scheme 1).

Pharmacology

The testing procedures for in vitro antibacterial, antifungal, minimum inhibitory concentration (MIC) and cytotoxicity bioassay have already been reported^14,15.

Results and discussion

Chemistry

Triazole derived Schiff base ligands (L¹)–(L⁵) were formed by the equimolar reaction of 3,5-diamino-1,2,4-triazole with a variety of methyl-, chloro- and nitro-substituted thiophene-2-carboxaldehydes (Scheme 1). All of them were air and moisture stable and soluble in dioxane, DMF and DMSO. All the oxovanadium(IV) complexes (1)–(5) were prepared by the stoichiometric reaction of VOSO₄ with respective triazole derived Schiff bases in molar ratio (M:L) 1:2 having a general formula, [M (L)₂]SO₄ where L = (L¹)–(L⁵) and M = V^IVO in a square-pyramidal geometry. All the synthesized oxovanadium(IV) complexes were air stable, nonhygroscopic and green-colored solids which decomposed on heating instead of melting. All oxovanadium(IV) complexes were only soluble in DMF and DMSO but not in common organic solvents. The elemental analysis and solubility data strongly recommended that these complexes are monomers and possessed 1:2 stoichiometry of the type ML₂. The analytical data given in Table 1, also, agreed well with the proposed structure of the complexes.

Table 1.  Physical and micronalytical data of the oxovanadium(IV) complexes (1)–(5).

No.	Complex	m.p. (°C)	Yield (%)	Calc. (Found) %
				C	H	N	V
(1)	(VO(L^1)2)SO4 (549.38)	262–264	84	30.60	2.57	25.27	9.27
	C14H14N10O5S3V (light green)			30.64	2.55	25.46	9.24
(2)	(VO(L^2)2)SO4 (577.43)	261–263	81	33.25	3.12	24.25	8.82
	C16H18N10O5S3V (deep green)			33.22	3.09	24.23	8.78
(3)	(VO(L^3)2)SO4 (577.43)	258–260	72	33.25	3.12	24.25	8.82
	C16H18N10O5S3V (sea green)			33.23	3.10	24.22	8.79
(4)	(VO(L^4)2)SO4 (618.27)	244–246	78	27.17	1.94	22.64	8.24
	C14H12Cl2N10O5S3V (light green)			27.15	1.92	22.61	8.22
(5)	(VO(L^5)2)SO4 (639.37)	281–283	82	26.30	1.88	26.28	7.97
	C14H12N12O9S3V (deep green)			26.27	1.86	26.23	7.94

Conductance and magnetic susceptibility

The molar conductance studies (Table 1) were carried out in DMF (1 mmol solution at 25°C). The high molar conductance data (82–90 ohm⁻¹ cm² mol⁻¹) of oxovanadium(IV) complexes (1)–(5) show them to have an electrolytic nature^16,17. The observed magnetic moment values of the complexes at room temperature were found to be in the range of 1.72–1.77 B.M. consistent with the values reported for a square-pyramidal geometry^18,19.

IR spectra

The most prominent IR frequencies of the triazole Schiff bases and their oxovanadium(IV) complexes are reported in the experimental part and Table 1. All the Schiff base ligands showed bands at 3343–3349, 3186–3194, 1609–1612 and 1018–1024 cm⁻¹, respectively, due to NH₂, N-H, C=N and N-N vibrations of triazole^20,21. Originally, a simple 3,5-diamino-1,2,4-triazole has two bands^22,23 at 3310 and 3350 cm⁻¹ assigned to two amino groups. All Schiff base ligands demonstrated an absence of a band at 3310 cm⁻¹ emerging into a new band of azomethine v(HC=N) linkage^3,24,25 at 1629–1637 cm⁻¹ giving thus an evidence for condensation of one amino group of the triazole moiety; however, another band of amino group at 3350 cm⁻¹ remained unchanged giving a clue of its non-participation in condensation reaction. The band appearing in all ligands at 882–889 cm⁻¹ was assigned²⁶ to (C-S) vibrations. Moreover, the ligands (L⁴) and (L⁵) showed vibrations at 810 and 1355 cm⁻¹ assigned to C-Cl and C-NO₂ groups, respectively. On comparison of the spectra of ligands with their oxovanadium(IV) complexes, the following conclusions can be drawn providing strong evidences for the complexation reaction between the ligands and the oxovanadium metal atom:

1. An azomethine, ν(HC=N) band at 1629–1637 cm⁻¹ in ligands shifted to lower frequency (10–20 cm⁻¹) at 1617–1621 cm⁻¹ in vanadyl complexes indicating coordination of azomethine-N with the vanadium (IV) metal ion²⁴.

2. The band at 882–889 cm⁻¹ assigned to ν(C-S) also shifted to lower frequency (10–15 cm⁻¹) at 866–875 cm⁻¹ indicating the coordination of thiophene-S with the vanadium(IV) metal.

3. The appearance of weaker low-frequency new bands at 487–497 and 371–388 cm⁻¹ attributed to v (V-N) and v(V-S), respectively, confirmed the coordination of triazole Schiff base ligand through azomethine-N and thiophene-S with the vanadium(IV) metal atom.

4. Two new bands which were not observed in the spectra of the ligands but appeared in the spectra of the complexes at 1082–1088 and 976–987 cm⁻¹ were assigned to the presence of (SO₄) outside the coordination sphere of the complex²⁷ and (V=O), respectively^28,29.

¹H NMR spectra

The ¹H NMR spectra of the triazole Schiff bases (L¹)–(L⁵) were recorded in DMSO-d₆. The ¹H NMR spectral data along with the possible assignments is recorded in the experimental part. All the heteroaromatic/aromatic protons were found as to be in their expected region³⁰. All the Schiff bases (L¹)-(L⁵) possessed a singlet because of amino (NH₂) proton, azomethine (CH=N) proton and NH protons of triazole moiety at 5.97–6.09, 8.32–8.68 and 11.92–12.20, respectively. Also, the (CH₃) protons of the Schiff bases (L²) and (L³) were found at 2.22–2.45. Moreover, the Schiff base (L¹) displayed the C₄-H proton of thiophene ring at 7.20 as a double doublet, C₃-H and C₅-H protons were observed at 7.71 and 7.81 as a doublet, respectively. In addition, the C₃-H and C₄-H protons of Schiff bases (L²), (L⁴) and (L⁵), also appeared as a doublet at 6.9–8.04. However, in case of the Schiff base (L³), the C₅-H proton was found at 7.68 as a doublet. Thus, the number of proton calculated from the integration curves³¹ and obtained values of the expected CHN analysis agreed well with each others.

¹³C NMR spectra

The ¹³C NMR spectra of the triazole Schiff bases (L¹)–(L⁵) were also recorded in DMSO-d₆. The ¹³C NMR spectral data along with the possible assignments is recorded in the experimental part. The ¹³C NMR spectra of the Schiff bases (L¹)–(L⁵) possessed triazole carbons (C₇) and (C₈) appeared in region at 160.85–166.24, respectively. The azomethine carbon (C₆) found in all the Schiff bases at 156.12–157.48. All carbons (C₂)–(C₅) of thiophene ring found in the Schiff bases (L¹)–(L⁵) were observed in the region at 122.63–149.97. In the Schiff bases (L₂) and (L₃), methyl carbons were observed in the region at 15.62–16.78. Moreover, the presences of the number of carbons are well in agreement with the expected values. Furthermore, the conclusions drawn from these studies present further support to the modes of bonding discussed in their IR and ¹H NMR spectra.

Electronic spectra

The electronic spectra of all the oxovanadium(IV) complexes (1)–(40) in DMF displayed three (Table 2) distinct low to high intensity bands (v₁, v₂ and v₃) which were assigned to b₂ (d_xy) → e_π(d_xz, d_yz), b₂ (d_xy) → b₁ (d_x² − _y²) and b₂ (d_xy) → a₁ (d_z²) transitions, respectively^32,33. The first band observed at 13269–13412 cm⁻¹ was assigned to b₂→ e_π d-d transitions. The second band was observed at 16473–16556 cm⁻¹ which can be attributed to b₂→ b₁ and the presence of third band at 24934–25087 cm⁻¹ can be assigned to the transitions b₂→ a₁. The fourth band of high intensity observed at 36175–37117 cm⁻¹ was due to metal → ligand charge transfer (MLCT). All these observations provide an evidence for the complexes to show a square-pyramidal geometry.

Table 2.  Conductivity, magnetic and spectral data of oxovanadium(IV) complexes.

No.	ΩM (Ω^−1 cm^2 mol^−1)	BM (µeff)	λmax (cm^−1)	IR (cm^−1)
(1)	82	1.75	13269, 18546, 26955, 36197	1619 (HC=N), 1088 (SO4), 976 (V=O), 865 (C-S), 419 (V-N), 375 (V-S)
(2)	84	1.74	13344, 18489, 26934, 36211	1621 (HC=N), 1082 (SO4), 978 (V=O), 867 (C-S), 415 (V-N), 371 (V-S)
(3)	83	1.73	13365, 18473, 26971, 36288	1620 (HC=N), 1085 (SO4), 980 (V=O), 872 (C-S), 416 (V-N), 375 (V-S)
(4)	86	1.72	13384, 18509, 27016, 36354	1620 (HC=N), 1086 (SO4), 984 (V=O), 870 (C-S), 427 (V-N), 380 (V-S)
(5)	89	1.77	13413, 18556, 27087, 36404	1617 (HC=N), 1083 (SO4), 987 (V=O), 874 (C-S), 434(V-N), 388 (V-S)

Biological activity

Antibacterial bioassay (in vitro)

In vitro antibacterial results of triazole Schiff bases and their oxovanadium(IV) complexes are reported in Table 3. All compounds were tested against four Gram-negative (Escherichia coli, Shigella flexneri, Pseudomonas aeruginosa, Salmonella enterica serover typhi) and two Gram-positive (Staphylococcus aureus, Bacillus subtilis) bacterial strains according to literature protocol^34,35. The obtained antibacterial results were compared with that of the standard drug imipenem, by considering its activity as 100%. The synthesized compounds showed different degree of inhibitory effects: low (up to 33%), moderate (up to 53%) and significant (above 53%). The (L₁) showed a significant (54–58%) activity against bacterial strains (a), (b), (d) and (f), and moderate (46–48%) activity against (c) and (e). Similarly, the (L₂) possessed significant (58–62%) activity against (b) and (e), moderate (38–50%) against (a), (c), (d) and (f). Also, the (L₃) demonstrated significant (55%) activity against (b), moderate (37–50%) activity was observed against (a), (c), (d) and (f), and weaker (31%) against (e). In addition, (L₄) exhibited significant (54–69%) activity against (a), (c), (d) and (e) except (b) and (f) which displayed moderate (48–50%) activity. Furthermore, (L₅) demonstrated overall significant (54–72%) activity against all bacterial strains (a)–(e). Beside this, the oxovanadium(IV) complexes (1)–(5) exhibited overall a significant (57–90%) activity against all bacterial strains. The results of these studies pointed out that the antibacterial activity is overall improved upon chelation/complexation that confirmed our previous studies^36–38. On comparison of the activity between Schiff base ligands and their oxovanadium(IV) complexes, higher activity was experienced by the compounds (4) and (5) owing to chloro and nitro substituents. “As these substituents are more electronegative therefore, they become involved in hydrogen bonding thus showing significant interaction with the bacterial proteins to enhance the bioactivity”. It is obvious from the above discussion that coordination makes the Schiff bases strongly antibacterial and inhibits the growth of bacteria more potentially rather than the uncomplexed Schiff bases.

Table 3.  Antibacterial activity (concentration used 1 mg/mL of DMSO) of triazole Schiff bases and oxovanadium(IV) complexes.

Bacteria						Compounds (Zone of inhibition (mm))
	(L^1)	(L^2)	(L^3)	(L^4)	(L^5)	(1)	(2)	(3)	(4)	(5)	SD
Gram-negative
(a)	16	13	16	17	19	19	16	18	24	26	29
(b)	17	14	18	15	18	21	18	21	19	25	31
(c)	14	17	14	17	19	18	19	17	20	24	30
(d)	16	12	15	17	21	20	16	19	23	26	29
Gram-positive
(e)	12	16	08	16	17	17	20	17	21	22	26
(f)	17	14	13	14	18	19	17	16	19	24	28
SA	1.83	1.97	3.41	1.26	1.37	1.41	1.63	1.79	2.10	1.52	1.72

(a), Escherichia coli; (b), Shigella flexneri; (c), Pseudomonas aeruginosa; (d), Salmonella enterica serover typhi; (e), Staphylococcus aureus; (i), Bacillus subtilis; 10< weak; >10 moderate; >16 and above significant; SD, standard drug (imipenem) SA, statistical analysis.

MIC for antibacterial activity

Compounds which showed significant (above 80%) antibacterial activity were selected for MIC studies. The obtained results are reported in Table 5. The MIC was determined using the disc diffusion technique by preparing discs containing 10, 25, 50 and 100 µg/mL of the compounds. It was observed that oxovanadium(IV) complexes (4) and (5) were found to be the most active due to chloro and nitro substitution. Amongst the two, compound (5) was found to be the most active showing the highest inhibition (3.891 × 10⁻⁸ M) against B. subtilis bacterial strain.

Antifungal bioassay (in vitro)

In vitro antifungal screening of all the synthesized compounds was carried out against Trichophyton longifucus, Candida albican, Aspergillus flavus, Microscopum canis, Fusarium solani and Candida glabrata fungal strains and results of inhibition were compared with the standard drugs, miconazole and amphotericin B. The results are reported in Tables 4 and 5. Ligand, (L₁) displayed significant (55%) activity against fungal strain (e), moderate (38–52%) against (a), (d) weaker (24%) against (b) and inactive against (c). Similarly, (L₂) showed significant (54–57%) activity against (b) and (f), moderate (38–50%) against (a) and (c)–(e) strains. On the other hand, (L₃) possessed significant (56%) activity against (b) and, moderate (41–47%) activity was experienced by (a), (d), (e) and (f) but (c) showed no activity. In the same way, (L₄) showed significant (55%) activity against (a), moderate (38–49%) against (c)–(f) and no activity was observed against (b). Furthermore, (L₅) showed moderate (45–48%) activity against (a), (e) and remaining possessed significant (54–65%) activity. The oxovanadium(IV) complex (1) exhibited significant (55–72%) activity against (a), (c)–(e) and moderate (42–48%) against (b) and (f). Besides this, compounds (2), (4), (5) possessed overall significant (54–78%) activity against all strains except (b) strain of complex (4) which showed moderate (38%) activity. The compound (3) displayed significant (56–70%) activity against (a), (b), (d) and (f), and moderate (35–49%) against (c) and (e), respectively. Complex (2) demonstrated significant (56–75%) activity against all strains except (d) which showed weaker (28%) activity. The complex (3) showed significant (56–70%) activity against (a), (b), (d) and (f), and moderate (35–42%) activity was observed against (c) and (e), respectively. It was generally observed that the activity of ligands was increased upon coordination with the vanadium metal.

Table 4.  Antifungal activity (concentration used 200 µg/mL) of triazole Schiff bases and oxovanadium(IV) complexes.

Organism						Compounds
	(L^1)	(L^2)	(L^3)	(L^4)	(L^5)	(1)	(2)	(3)	(4)	(5)	SD
(a)	45	50	44	55	48	61	63	58	69	63	A
(b)	24	57	56	00	55	48	72	70	38	72	B
(c)	52	48	00	44	60	63	59	35	56	78	C
(d)	43	38	41	46	58	55	58	56	60	75	D
(e)	55	43	50	38	45	67	56	42	55	61	E
(f)	00	54	47	29	44	42	68	63	64	70	F
SA	20.91	7.00	20.11	19.56	6.89	9.55	6.25	13.13	5.65	6.66

(a), Trichophyton longifucus; (b), Candida albican; (c), Aspergillus flavus; (d), Microscopum canis; (e), Fusarium solani; (f), Candida glabrata; SD, standard drugs MIC µg/mL; A, miconazole (70 µg/mL: 1.6822 × 10⁻⁷ M/mL); B, miconazole (110.8 µg/mL: 2.6626 × 10⁻⁷ M/mL); C, amphotericin B (20 µg/mL: 2.1642 × 10⁻⁸ M/mL); D, miconazole (98.4 µg/mL: 2.3647 × 10⁻⁷ M/mL); E, miconazole (73.25 µg/mL: 1.7603 × 10⁻⁷ M/mL); F, miconazole (110.8 µg/mL: 2.66266 × 10⁻⁷ M/mL). SA, statistical analysis.

Table 5.  Minimum inhibitory concentration (M/mL) of the selected compounds (4) and (5) against selected bacteria.

Microorganisms	MIC (M/mL)^a (4) (5)
Escherichia coli	2.323 × 10^−6	2.324 × 10^−6
Shigella flexneri	ND	ND
Pseudomonas aeruginosa	ND	ND
Salmonella enterica serover typhi	2.342 × 10^−6	3.212 × 10^−7
Staphylococcus aureus	3.179 × 10^−5	2.127 × 10^−7
Bacillus subtilis	1.672 × 10^−7	3.891 × 10^−8

^aEach compound was measured three times, with a standard deviation (SD) less than 10% in all the cases. ND is not determined at tested concentrations.

Cytotoxic bioassay (in vitro)

All synthesized compounds were screened for their cytotoxic activity (brine shrimp bioassay) activity using the protocol of Meyer et al.^39,40 The compound (2) only exhibited effective cytotoxicity (LD₅₀ = 2.154 × 10⁻⁴) against Artemia salina, while all other compounds were almost inactive for this assay.

Conclusion

It has been demonstrated that the complexation of bioactive triazole ligands with oxovanadium(IV) form a new set of antimicrobial agents with an attractive range of efficiency against the aforesaid microorganisms. Particularly, the MIC values of compound (4) and (5) and level of cytotoxicity against compound (2) are significant and encouraging considering the current existence of drug-resistant strains in various clinical practices. Moreover, the present studies have also indicated that the formation of the oxovanadium(IV) complexes plays a key role in determining antimicrobial properties.

Acknowledgment

One of the authors (SHS) is grateful to Higher Education Commission (HEC), Government of Pakistan for the award of scholarship to carry out this research. We are also indebted to HEJ Research Institute of Chemistry, University of Karachi, Pakistan, for providing their help in taking the NMR, mass spectra, antibacterial, antifungal and brine shrimp bioassays.

Declaration of interest

The authors report no conflict of interest and are responsible for the contents and writing of the paper.